This invention generally relates to methods and devices for making electrical connections between two electrically conductive elements, specifically to electrical connections between electrically conductive elements in acoustic transducers.
Electrical connections to the piezoelectric ceramic in ultrasound transducers must meet requirements of low electrical resistivity and high bond strength while not impacting the acoustic performance. The piezoelectric ceramic in ultrasound transducers is often bonded to a flexible printed circuit board (hereinafter xe2x80x9cflex circuitxe2x80x9d) or other electrical connector using an epoxy bond. This bond joint needs to be electrically conductive for the applied voltage to be converted to mechanical energy by the piezoelectric material. However, bonding epoxy in an unfilled state is an electrical insulator. Therefore an epoxy bond that prevents intimate contact between the ceramic and flex circuit will result in poor electrical contact. Epoxy formulations possessing conductive fillers are known to provide electrical conduction between electrically conductive surfaces, however these materials possess filler particles in a sufficiently high concentration as to cause acoustic reflection in thin layers when placed within the acoustic path of an ultrasound transducer.
Many ultrasonic transducers are phased arrays comprising single or multiple rows of electrically and mechanically independent transducer elements. In these types of transducers, each transducer element may be a layered structure comprising an acoustic absorber, a piezoelectric ceramic layer, one or more acoustic matching layers, and a front wear plate or focusing lens. Typically, one or more flex circuits are used to make electrical connections (signal and ground) from the piezoelectric ceramic layer to the signal processing electronics, or to a bundle of coaxial cables that ultimately connect to the signal processing electronics.
To obtain a bond with sufficiently low electrical resistivity for ultrasound transducers, a high pressure is commonly applied to the bond interface prior to and during curing of the epoxy. If the flex circuit and piezoelectric ceramic surfaces are microscopically rough and the epoxy layer is sufficiently thin, then an electrical connection is achieved via a distribution of direct contacts between high points on the piezoelectric ceramic surface and high points on the flex circuit. Under sufficient pressure, metallized asperities on the ceramic penetrate through the epoxy bond to make direct contact with the flex circuit. Thin epoxy bonds can provide low electrical resistivity and acceptable acoustic properties, however the bond strength is reduced for such thin bonds. In accordance with this technique, the actual contact-area through the epoxy layer, and hence the electrical conductivity, varies with the ceramic surface texture and relative degree of parallelism of the two materials. Epoxy bond joints are known to be relatively weakened when applied in very thin layers. Commercial epoxy suppliers often recommend a minimum bond thickness of 10 microns or greater. The high-pressure bonding process needed for electrical reasons results in an epoxy bond that is by necessity thinner than desired. Therefore, for optimum adhesion it is desired to have a thicker flex circuit-to-ceramic bond while for optimum electrical contact and acoustic performance the bond should be thin. These two requirements are directly opposed to one another.
The above-described aspects complicate the ultrasound transducer manufacturing process. The ceramic and flex circuit surfaces need to be flat and parallel, and bonding pressures need to be high to prevent formation of a thick, and hence not electrically conducting, epoxy bond. There is a need for a simpler manufacturing technique that satisfies the dual requirements of optimum adhesion and optimum electrical contact.
The present invention is directed to a method of making an electrical connection between an ultrasonic transducer element or array of elements and a conductive trace of a printed circuit (including inflexible as well as flexible printed circuits). The invention is also directed to the resulting assembly.
One aspect of the invention is an assembly comprising: a layer made of ultrasound transducing material; an electrically conductive coating on a surface of the layer of ultrasound transducing material; a substrate made of dielectric material; an electrical conductor formed on a surface of the substrate, the substrate being disposed so that the electrical conductor confronts the electrically conductive coating on the layer of ultrasound transducing material; an electrically conductive mesh disposed so that a portion intervenes between and is in contact with the electrical conductor on the substrate and the electrically conductive coating on the layer of ultrasound transducing material; and adhesive material occupying interstices in the mesh and in contact with the electrically conductive coating and the electrical conductor: For example, the mesh may comprise an electroformed metal mesh or a metal-plated polymeric mesh.
Another aspect of the invention is an ultrasound transducer comprising: a body of piezoelectric ceramic material, the body comprising front and rear surfaces; an electrode formed on the rear surface of the body of piezoelectric ceramic material; a substrate made of dielectric material; a pad of electrically conductive material formed on a surface of the substrate, the substrate being disposed so that the pad confronts the electrode; an electrically conductive mesh disposed so that a portion intervenes between and is in contact with the pad and the electrode; and adhesive material occupying spaces in the mesh and in contact with the pad and the electrode.
A further aspect of the invention is an ultrasound transducer comprising: an array of ultrasound transducer elements, each of the ultrasound transducer elements comprising a respective body of piezoelectric ceramic material and a respective electrode formed on a surface of the respective body, the bodies being substantially acoustically isolated from each other, and the electrodes being substantially electrically isolated from each other; and a printed circuit comprising an array of pads of electrically conductive material, each pad confronting a respective one of the electrodes, the pads being substantially electrically isolated from each other. The printed circuit is bonded to the array of transducer elements by adhesive material disposed between the confronting electrodes and pads. The transducer further comprises a multiplicity of sections of an electrically conductive mesh embedded in the adhesive material, each one of the mesh sections being sandwiched between a respective one of the electrodes and a respective one of the pads. Each mesh section is separated from adjacent mesh sections by a respective gap.
Yet another aspect of the invention is a method of making an electrical connection between a pair of electrically conductive surfaces, comprising the steps of: placing an electrically conductive mesh and a mass of adhesive material between a pair of mutually opposing electrically conductive surfaces; pressing the electrically conductive surfaces together with the mesh and adhesive material therebetween with sufficient pressure that the mesh contacts the electrically conductive surfaces; and curing the adhesive material while maintaining the electrically conductive surfaces in a pressed state.
Another aspect of the invention is a method of assembling an ultrasound transducer, comprising the following steps: (a) metallizing a surface of a layer of piezoelectric ceramic material; (b) metallizing a surface of an dielectric substrate in accordance with a pattern; (c) arranging the piezoelectric ceramic layer, the dielectric substrate, an electrically conductive mesh and a mass of adhesive material-so that the metallized surface of the piezoelectric ceramic layer and the metallized surface of the dielectric substrate confront each other, and the mesh and the adhesive material are disposed between the confronting electrically conductive surfaces; (d) pressing the piezoelectric ceramic layer and the dielectric substrate together with the mesh and adhesive material therebetween with sufficient pressure that the mesh contacts the electrically conductive surfaces; and curing the adhesive material while maintaining the piezoelectric ceramic layer and the dielectric substrate in a pressed state.
A more robust bonding process is obtained by placing a thin electrically conductive mesh at the bond area. Such a mesh improves electrical conductivity at lower bonding pressures and can compensate for non-parallel bond surfaces while the thicker epoxy bond joint formed within the mesh openings increases the bond strength. Metal meshes only a few microns thick and having low metal density are sufficiently close to being acoustically transparent that they do not affect transducer performance. The electrically conductive mesh is placed in the bond area to provide electrical connection between the metallized ceramic and flex circuit substrates. Because of the openings in the mesh, a thicker, hence stronger, epoxy bond is formed.
Other aspects of the invention are disclosed and claimed below.